Now I've read from numerous sources that you start to fertilize when you see four leaves. Now is that four leaves or four sets of leaves( 8 actual leaves ) minus cotyledons of course. Maybe I'm just smoking too much into this. Also I have three seeds going(DIY bubbleponics) They were planted at the same time. One has 2 sets of leaves. one is retarded, the cotyledons never really opened and has only 1 true leaf and the other one the tap root is growing over the top of the seed and it hasn't broke the surface of the rooter plug. Will it grow up? With that said, should I wait on the fertilizing until the all have the correct # of leaves?
2 weeks after sprout comes up you can give them a light dose..25% mix then gradually take it up over a few weeks... unless they are two weeks old they just need plain ph balanced water.
most sources I have found here in town (G.C.) seem to lean toward putting your plant(s) on a mild dose of ferts. (i.e: My fertilizer calls for 1tsp mix for a gallon H2o, but I do 1/2 tsp/per gal. H2o) I'm still beginner, and my plants have been on ferts for three days and they seem to LOVE it. Rapid healthy growth.
I would recommend 2 or 3 weeks after they pop out of the soil to start ferting. And like Budslinger said, start with 1/4 strength or even less. Under ferting is much easier to cure than over ferting. I waited about 4 weeks before I started ferting and they were showing pretty serious deficiency, so don't wait that long.